The property market in London is continuously changing, and more people choose to renovate their houses instead of changing their addresses especially with the exorbitant transaction rates, high stamp duty and space constraints. House extension is one of the most popular methods of adding space to live in, improving lifestyle, and improving property value. However, prior to getting on paper or talking to contractors, the one question that most homeowners are asking themselves is: How much will it really cost?

This guide separates out the actual expenses that you will likely face in the existing market of London, the determinants in pricing, typical project scenarios, and the budgeting process in order to extend your project.

 

1. Why Londoners Are Choosing Extensions

The reasons why London homeowners opt to do extensions vary:

• Design bigger open plan kitchen and dining areas.
• Additional bedrooms or bathrooms.
• Construct utility rooms or home offices.
• Make the most of space and do not leave a favourite neighbourhood.
• Boost property value

Expanding your home in most instances may prove to be far more economical than selling and purchasing another house in the luxurious London market- especially when stamp duty, agents fees and relocation expenses are taken into consideration.

 

2. Typical Price Ranges for House Extensions in London

BBS Construction UK’s Estimated Cost Guide

According to estimates referenced from How House Extensions Are Priced in London (which draws on real London project data and industry pricing), the approximate costs for different types of extensions are:

Extension Type	Average Cost per m²	Typical Total (20m²)
Rear Extension	£1,800 – £2,800	£36,000 – £56,000
Side Return Extension	£2,000 – £3,000	£40,000 – £60,000
Wrap-Around Extension	£2,500 – £3,500	£50,000 – £70,000
Double-Storey Extension	£2,800 – £4,000	£60,000 – £80,000
Loft Extension	£1,500 – £2,500	£30,000 – £50,000
Basement Extension	£3,500 – £5,000	£70,000 – £100,000+

These are average 2025 prices and will change determined by the complexity of designs, level of specification and on-site conditions. Never run out of budgeted funds; add 10-15%, contingency fee, to your budget to cover unexpected expenses (drainage or structural changes).

 

Other Recent London Cost Data

The guides of independent market confirm the same level of costs in the city:

• Standard London extensions often average £3,100–£4,300 per m² for a good-quality build, with project totals for ground-floor or rear extensions typically between £55,000 to £120,000.
• In 2026, average London extension prices usually sit around £2,500 – £4,500 per m², depending on finish quality and extension style.
  

 

 

3. What Determines the Cost of Your Extension?

a. Size and Shape of the Extension

The easiest thing is the size of the extension. Extensions that are larger tend to be more expensive; and more difficult shapes of footprint (such as wrap-around ones) require more structural work and more materials.

 

b. Type of Extension

Budgets of different styles are very different:

• Rear Extensions — This extension is used in kitchen/diner extensions and the use of an open plan.
• Side Return Extensions — These are ideal with terraced houses, and access can add to the costs.
• Double-Storey Extensions — Add more space – suitable when adding additional bedrooms – more expensive because of structural and roofing.
• Loft and Basement Extensions — Loft extensions and basement extensions can be economically viable to add space but require expert work and structural implications.

 

c. Quality of Materials and Finishes

More expensive finishes, custom made joinery, under floor heating, higher quality windows or bi-fold doors will increase the costs – in some cases very high cost.

 

d. Professional Fees

Professional expenses not related to the construction should not be underestimated:

• Architect or designer services
• Structural engineers
• Planning applications
• Party Wall agreements (if walls adjoin a neighbour)

These payments can increase your overall budget by several thousand pounds and more.

 

e. Site Conditions & Access

Complex locations (narrow aisles, strenuous ground, bad soil, limited material movements) increase labour and time, which affect the overall cost.

 

f. VAT and Regulations

The extension work done in London on homes is liable to 20% VAT which must be incorporated in your contractor expenses unless there are special exemptions.

 

4. Example Costs You Might Expect

Let’s take two real-world examples:

Example 1: 20m² Rear Single-Storey Extension

• Construction cost (mid-range): ~£2,800/m²
• Build cost total: £56,000
• Professional fees: £5,000
• VAT (20% on construction): £11,200
  Total Project Budget: ~£72,000+

 

Example 2: 30m² Wrap-Around Extension

• Construction cost (higher spec): ~£3,000/m²
• Build cost total: £90,000
• Professional fees + Planning: £8,000
• VAT (20%): £18,000
  Total Project Budget: ~£116,000+

These are illustrative figures — exact costs will vary significantly based on options chosen.

 

5. Is an Extension Worth the Investment?

Although the initial investment is high an extension when reasonably done would add value to the property which in most cases would be a huge portion of the cost of the project when you later come to sell. Extension of kitchen and open-plan space features are likely to give some of the highest returns.

In most locations around London, a new high-quality extension will open up space to live without necessarily relocating, remaining in your locality and enhancing your house.

 

6. Money-Saving Tips for Your London Extension

• Tend to builders early enough and must have clear drawings.
• Get several builders to quote — competitive prices are helpful.
• Consider progressive finishes when finances are constrained e.g. full structure now, interiors later.
• Think about designing and building contracts that may simplify the charges and mitigate surprises.
  

 

Closing Thoughts

The cost of a house extension London is an open question that has no one-size-fits-all answer, however with the current price information and actual cost scales at hand you can begin budgeting with some confidence.

Regardless of whether you are intending to build a rear-kitchen-extension, a double-storey-add-on, a side-return, or a loft/basement, it is essential to learn how the prices are organized and what factors they are influenced by. Begin with expectations, make intelligent plans, contingency plans are necessary – and professional advice will help to keep your London extension project on schedule and on a tight budget.

Are you prepared to investigate your alternatives? Request a free quote with any of the reputable London extension builders – the best course of action is to get precise estimates that are based on your plans and property.